I. Field of the Invention
The present invention relates to apparatus for exciting waveguide type gas lasers.
II. Description of the Prior Art
Waveguide type gas lasers have been developed in which a laser beam is generated by the excitation of gas located within a hollow waveguide passageway. There are presently two basic excitation techniques for waveguide type laser apparatus. One of these known techniques is conventional DC excitation which involves application of a DC voltage on the order of 10 KV to the electrodes of a waveguide type gas laser. The second of these known techniques involves radio frequency excitation in the VHF or UHF range of 30 Mhz to 3 Ghz.
The above-mentioned DC excitation technique may be classified into two varieties depending upon whether the path of discharge between the electrodes is parallel to the light axis of the laser (lengthwise excitation) or perpendicular to that axis (transverse excitation). An example of a discharge path between electrodes being parallel to the light axis of a laser is illustrated in FIG. 1. More particularly, in FIG. 1 a light axis 3 is shown located within a waveguide passageway 2 which is defined by electrodes 1 and an insulation housing 5. Electrodes 1 are located along passageway 2 such that when a discharge occurs between electrodes 1, the discharge occurs parallel to the direction of light axis 3.
In FIG. 2 there is illustrated a light axis 3 located within a passageway 2. In FIG. 2 electrodes 1 are disposed facing one another on either side of light axis 3. Accordingly, any discharge between electrodes will follow a path generally perpendicular to the direction of light axis 3.
Either arrangement illustrated in FIG. 1 or FIG. 2 allows a discharge between electrodes 1 and allows light resulting from that discharge to be reflected between resonant mirrors (not shown) disposed at the ends of passageway 2 to cause oscillation and thereby permit emission of an amplified laser beam.
However, in the excitation arrangement illustrated in FIG. 1, since the distance between electrodes 1 is long, such a system requires a high voltage be applied between electrodes 1 in order to achieve satisfactory discharge of gas located in passageway 2. Moreover, in such systems electrodes 1, passageway 2, and the resonant mirrors are put in a laser tube which is hermetically sealed to contain gas under pressure. The pressure of the gas is high to maximize the lifetime of the apparatus. However, this high gas pressure increases the voltage requirement to achieve satisfactory discharge.
The arrangement shown in FIG. 2, on the other hand can be used with a lower application voltage than the lengthwise excitation system of FIG. 1 since, as shown in FIG. 2, the faces of electrodes 1 are parallel to one another and, therefore, are positioned at a short distance from one another. Moreover, although the transverse excitation system as illustrated in FIG. 2 has an advantage in that such a system permits use of increased gas pressure without undue increase in required voltage levels, the transverse excitation system of FIG. 2 has a disadvantage in that it is difficult to achieve uniform discharge in passageway 2 due to the existence of parallel faced electrodes 1.
Furthermore, a common disadvantage of both DC excitation systems of FIGS. 1 and 2 is that such systems require connection to a ballast resistance in series with electrodes 1, thereby decreasing efficiency and increasing the requisite size of the apparatus. Moreover, both systems are subject to very large voltage increases in the vicinity of the cathode known as a "cathode fall" phenomena. This phenomena results in spattering of cathode electrodes and adjacent mirrors, which in turn results in a decrease in laser output. The high resultant electric field in the area of the cathode also causes separation of laser gas which tends to shorten the life of the apparatus.
DC excitation waveguide gas lasers are disclosed for example, in U.S. Pat. No. 3,772,611 issued to Smith and 4,103,255 issued to Schlossberg. Although such DC excitation systems had the advantage of employing a power supply with comparatively simple construction, such systems require a lengthened waveguide pass or discharge pass for acquiring a laser beam of large output. Accordingly, in a lengthwise excitation system, the problem of providing the requisite discharge voltage is increased since the interval between electrodes is increased. In the transverse excitation system a problem with respect to efficiency develops because of the difficulty of providing spacial uniformity of discharge between the electrodes.
In view of the foregoing problems, the output from a waveguide type gas laser apparatus utilizing DC excitation is generally on the order of a few watts at the most.
A high-frequency excitation system is shown in FIG. 3 as comprising a pair of electrodes 1 disposed in parallel to face one another. Electrodes 1 are separated by insulators 4. Electrodes 1 and insulators 4 operate to define a waveguide passageway 2. A high-frequency or radio frequency field may be developed in passageway 2 in the VHF or UHF zone of 30 Mhz to 36 Ghz by application of such radio frequency signals to electrodes 1. A radio frequency excitation system such as that disclosed in FIG. 3 has the following advantages:
1. A greater life expectancy than in a D.C. excitation system due to lack of mirror contamination and gas separation by removing the "cathode fall" phenomena found in DC excitation systems.
2. A reduction in the loss of excitation energy by the removal of "cathode fall."
3. A reduction in the size of the power supply since the excitation voltage can be as low as 100 volts if the electrodes are located in a transverse orientation as shown in FIG. 3.
4. Elimination of the need for a ballast resistor to achieve discharge stabilization due to positive discharge impedance, thereby reducing the size and increasing the efficiency of the apparatus.
5. A greater uniformity of spatial distribution of discharge.
6. A higher output from a smaller and lighterweight apparatus.
However, radio frequency excitation systems are subject to the following disadvantages:
1. The requirement of having a radio frequency excitation source in addition to a DC voltage supply.
2. The requirement of having impedance matching between the waveguide passageway and the radio frequency excitation source.
3. Substantial heat generation in the radio frequency excitation source due to conversion inefficiency from DC to radio frequency signals on the order of about 60%, depending upon the frequency involved.
4. The inability to achieve a broad frequency shift due to the need to maintain matching of radio frequency signal with the waveguide passageway so that a wide range of laser output cannot be achieved even with frequency modulation. Moreover, even if the applied high frequency voltage is allowed to change by amplitude modulation, a large loss would result in the radio frequency supply circuit, thereby causing the generation of substantial amount of heat and making it difficult to produce a small and lightweight apparatus due to the need to provide for substantial heat dissipation.
It is therefor an object of the present invention to provide a waveguide type gas laser excitation apparatus which eliminates the disadvantages of radio frequency excitation apparatus while maintaining the advantages of DC excitation systems.
More specifically, it is an object of the present invention to provide a highly efficient, compact waveguide type gas laser excitation system.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or maybe learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.